Get the automatic.
Like Oldcat have said, with a manual PFD, if you go in disabled, then you are dead. High humidity, huh....it will have to be very high humidity, or significant water spray, could trigger it. But, the trigger is usually covered with a little flap from rain or water spray. If you are concerned with high humidity, you can get the rearming kit, and use just the tablet thingy, looks like a big aspirin tablet. Some people have said you can use aspirin tablet as a substitute, but I have never tried that, so I don't know if that will really work or not.